In application areas such as metropolitan area networks, local area networks, fiber-to-the-home networks, and medium- and short-haul communications, to facilitate laying or reducing the occupied space, an optical fiber cable often needs operate at a small bending radius. However, a conventional single-mode fiber cannot meet the requirement of operation at a small bending radius. To meet the requirement of the low bending loss of the optical fiber, the international G.657 standard is proposed for the optical fiber. The low bending loss of the optical fiber is usually implemented by way of reducing the size of the core of the optical fiber, introducing a cladding structure having grooves, or using an aperture-assisted optical fiber (K. Himeno, S. Matsuo, N. Guan, and A. Wada, “Low-Bending-Loss Single-Mode Fibers for Fiber-to-the-Home,” Journal of Lightwave Technology, 2005, 23(11): 3494-3499).
Although a G.657 optical fiber can operate at a bending radius of 7.5 mm or even 5 mm, the bending loss of the optical fiber is still large, so it is difficult for the G.657 optical fiber to stably operate at a small bending radius for a long time. For example, the G.657B3 optical fiber should have a bending loss less than 0.08 dB/turn at a bending radius of 7.5 mm and 0.15 dB/turn at a bending radius of 5 mm (1550 nm wavelength). Apparently, after being wound for several coils at this small bending radius, the optical fiber still has a large loss that impacts the performance of the communication system.
According to the optical fiber theory, for a step index optical fiber, when the normalized frequency is less than 2.405 at the operating wavelength, the optical fiber is a single-mode optical fiber; when the normalized frequency is greater than 2.405, the optical fiber can transmit a high-order mode and thus serves as a non-single-mode optical fiber. A conventional multi-mode or few-mode optical fiber has a high-order mode that causes serious intermodal dispersion in transmission of signal light through the optical fiber, limiting the communication rate and capacity of the optical fiber. Thus, single-mode optical fibers still serve as main transmission media in an optical fiber communication system at present.
If a non-single-mode optical fiber is used in the optical fiber communication system, the difference between the refractive index of the core and the refractive index of the cladding of the optical fiber is effectively increased, so that transmission with a low bending loss can be achieved; and the non-single-mode optical fiber is matched with and connected to a single-mode optical fiber, so that single-mode transmission can be achieved (Chinese patent No. 201010589018.1 entitled “OPTICAL FIBER COMMUNICATION SYSTEM” and Chinese patent No. ZL201210393511.5 entitled “COMMUNICATION SYSTEM BASED ON FEW-MODE OPTICAL FIBER”). This technology reduces limits on the number of optical fiber transmission modes and implements connection to the single-mode optical fiber to suppress the generation of high-order modes, thereby meeting the requirements of low bending loss, single-mode transmission and low connection loss. However, this method requires that the few-mode optical fiber be connected to single-mode fibers at both ends with small connection deviation. This limits the practical use of the few-mode optical fiber.
Therefore, it is required to provide an optical fiber capable of single-mode transmission and, after being wound for several coils at a small bending radius, capable of low-loss transmission.